Eukaryotic cells contain a number of small GTPases that act as intermediates in signal transduction. The Ras family of GTPases includes Ras and Ras-like proteins, which control of cell growth and differentiation, the Rho and Rac proteins that regulate cytoskeletal organization, and the Rab and Ran proteins that regulate vesicular sorting. Ras proteins play an important role in the regulation of cellular proliferation through intracellular signaling pathways in which binding of an extracellular signal molecule to a tyrosine kinase receptor is transmitted to the cell nucleus, resulting in the initiation of transcription of specific genes.
Similar to other guanine-binding proteins (such as the heterotrimeric G proteins), the Ras proteins cycle between an active guanosine-triphosphate (GTP) bound form and an inactive, guanosine-diphosphate (GDP) bound form. The balance of these two states determines the decision to undergo division. The weak intrinsic GTPase activity of Ras proteins is greatly enhanced by the action of GTPase activating proteins (GAPs). Point mutations have been described in Ras genes (`activating` or oncogenic mutants) that decrease the intrinsic GTPase activity of Ras and render it insensitive to stimulation by GAPs. GAP genes are also known for the other families of small GTPases.
Tumor susceptibility genes may be oncogenes, which are typically upregulated in tumor cells, or tumor suppressor genes, which are down-regulated or absent in tumor cells. Malignancies may arise when a tumor suppressor is lost and/or an oncogene is inappropriately activated. When such mutations occur in somatic cells, they result in the growth of sporadic tumors. Familial predisposition to cancer may occur when there is a mutation, such as loss of an allele encoding a tumor suppressor gene, present in the germline DNA of an individual.
Abnormal signal transduction involving activated Ras genes plays a major role in the development of a variety of tumors. Ras genes are mutated in approximately 30% of all human tumors, suggesting a key role in a number of different cell types. GAP proteins may also have a role in abnormal growth control, because they increase GTPase activity in the bound protein, thereby shifting the balance in the cell to the inactive, GDP binding form. GAP activity has been demonstrated to result from a catalytic domain termed the GAP Related Domain (GRD). Neurofibromin, the product of the NF1 gene contains a GRD and possesses GAP activity towards ras subfamily members. It has been shown that a predisposition to neurofibromatosis is correlated with mutations in the NF-1 gene.
More recently a family of RasGAPs have been identified that share multiple conserved motifs: two C2 domains, homologous to the C2 regulatory region of protein kinase C, a GRD and a pleckstrin homology domain. This arrangement was first observed in the Drosophila GAP1 gene and has since been identified in four mammalian GAPs, rat GAP1.sup.m, human GAP1.sup.IP4BP, bovine P98GAP, and mouse GAPIII. These proteins can be collectively grouped as the rasGAP1 family. Outside of the GRD, the p120GAP, NF1 and GAP1 proteins are dissimilar and possess additional motifs that may serve to regulate GAP catalytic activity or the interaction of these proteins with other cellular signaling molecules.
The involvement of GAP proteins with regulation of Ras and Ras-like protein activity, and the association with tumor development makes the further identification of novel GAP genes of great interest.